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T O P I C R E V I E W

Shikedants

This question may sound really dumb, yet it's something that I wonder about.

The Apollo lunar module (LM) descended to the lunar surface in the vacuum of outer space. The LM had to emit significant amounts of exhaust during its descent. Where did that exhaust go? Does it just go poof, gone?

I mean exhaust is tangible and there is no wind or other atmospheric condition that could dissipate it yet gravity would cause these molecules to settle on the surface, no? Could there possibly be a cloud of exhaust in the midst of the LM and the moonwalkers?

What happens to any exhaust emitted in outer space? Such as in earth orbit.

randy

The exhaust did impact on the lunar surface. If you watch the landing sequence of the various landing missions, you can see the dust being kicked up by the exhaust and flying off in all directions. Then as soon as the engine is shut down, the dust settles.

ilbasso

Since the lunar vicinity is essentially a vacuum, the exhaust gases rapidly dissipate.

Armstrong noted in the post-flight briefings that as the LM landed, he could see dust and small rocks being accelerated by the LM exhaust, flying out so rapidly that they "disappeared over the horizon" before he shut down the engine.

Shikedants

Randy and Jonathan, thanks for responding.

I know you can see from the videos the effects of the exhaust what with all the dust flying. I think you may have partially answered my question with Neil's comments that the dust and rocks went over the horizon.

But still I'm wondering what happens to the actual gasses that have been emitted from the spacecraft.

Does a molecule of spent aerozine (or whatever fuel it was) cease to exist. It has to go somewhere doesn't it?

Mr. Armstrong said it could have been pushed over the lunar horizon. Are there molecules of aerozine still present on the lunar surface.

Are there clouds of exhaust gasses in orbit around the Earth, Moon or Mars? If so, would these clouds cause spececrafts' orbits to slowly degrade over time?

Headshot

To clear up a misconception. The moon does have a gaseous envelope around it, but this envelope does not meet the definition of "atmosphere." Altogether, it would weigh about 10,000 kg. It is constantly dissipating into space while also being replenished by various means having nothing to do with lunar landings.

When a LM landed, most of its exhaust went off into space and some (between 20% to 30%) became part of this gaseous envelope ... temporarily. Sunlight would break the exhaust molecules down into their lighter components, which would eventually escape the lunar environment.

Also adding to the envelope would be outgassing from the remaining propellant in the tanks of the LM descent stage and the various gases in the tanks of the astronauts PLSSs (when they were using them and when they were discarded on the lunar surface). Also, the ascent engine's exhaust made a smaller contribution.

These "contributions" were measurable by certain ALSEP experiments.

I believe an Apollo 12 ALSEP experiment even detected the exhaust from Apollo 14s Antares LM during its ascent phase (Antares almost passed over the 12 landing site during its ascent). I remember reading an estimate of how much exhaust, in kgs, remained in the lunar environment (temporarily), but do not recall the figure. It might be in one of the Apollo Preliminary Science Reports.

Whether or not any LM exhaust molecules remain in the lunar environment would be a statistical exercise.

Shikedants

I wonder no more. Thanks!!!

Jim Behling

quote:Originally posted by Shikedants:Are there clouds of exhaust gasses in orbit around the Earth, Moon or Mars? If so, would these clouds cause spececrafts' orbits to slowly degrade over time?

The gases around Earth and Mars are called the atmosphere. Some of the exhaust gases become part of it. Per wiki:

Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter.

This is where most of the gases go.

YankeeClipper

The lunar module descent and ascent engines both used a hypergolic liquid bipropellant system comprised of:

The primary combustion products of Aerozine-50 and Nitrogen Tetroxide consist of H2O (Water Vapour), N2 (Gaseous Nitrogen), CO (Carbon Monoxide), CO2 (Carbon Dioxide), H2 (Gaseous Hydrogen), and OH (Hydroxyl Radical). Over 50 secondary chemical reaction products have been reported and more complete details of the chemistry can be found here.

The small size, reactivity, and volatility of the propellant molecules and the nature of the chemical reaction products mean that they will not remain long in a low-gravity environment bombarded with intense electromagnetic radiation and thermal energy.

Max Q

I knew that the moon has a thin gaseous envelope but reading this thread makes me wonder what is its make up. Has it ever been analysed?

Headshot

I believe experiments conducted on Apollos 12 and 17 found that three gases make up the bulk of the moon's gaseous envelope: neon, helium, and hydrogen (in roughly equal amounts).

Small amounts of carbon dioxide, ammonia, and methane were detected as well. Also, argon-40 was detected, its abundance increased at times of major lunar seismic activity. Argon-40 is created by the radioactive decay of potassium-40 in the lunar interior. Apparently, seismic activity allows argon-40 to escape from the lunar interior and reach the surface via newly-created fractures.